You're now able to recover a deleted Google Site by going to its old URL within 30 days of deletion. Note you won't be able to recover any site you deleted before 12 April 2011, and only the account that originally created the site can recover it.
When you delete a site you will see:
This site will be immediately removed from view but you may recover it by visiting its URL within 30 days. Are you sure you want to delete this site?
After you delete the site, if you access the URL of the site while signed in with the same account that you used when you deleted it you will see:
The webspace [site name] was deleted by [account name] on [date and time deleted] and you will have buttons to recover the site or cancel.
If you want to reuse the site URL for a new site without recovering the old one you must permanently delete the site. Go to sites.google.com and look at your deleted sites then permanently delete your site. After the site is permanently deleted you can create a new site using the same URL. You can only reuse the URL when using the same account that originally created it.
You also now have an option to recover deleted pages from your site. See http://www.google.com/support/sites/bin/answer.py?hl=en&answer=187383